In typical refinery processes, stored heavy crude oil is cleaned of contaminants (e.g., sand, salts and water) as the first step in the refining process by passage through desalting units. The clean crude feedstock is then heated by passing the desalted crude through a series of heat exchangers. The crude is then passed through a furnace that heats the crude oil to a higher temperature. The furnace, which may be an oil, natural or refinery fuel gas-fired furnace or electrically fired furnaces, heats the oil and is injected into an atmospheric distillation tower. The extreme heat produces physical splitting of the crude oil into combustion gas (furnace fuel gas) and other gaseous light ends, liquid products, and an atmospheric resid fraction.
A large amount of heavy resid content is characteristic of heavy oils. The atmospheric resid must be subjected to more refining. Following the atmospheric tower, the resid is further heated in another series of heat exchangers and then another furnace and sent to a vacuum distillation tower, where light vacuum gas oil and heavy vacuum gas oil are extracted from the resid. The remaining tarry fluid left near the base of the vacuum tower, the vacuum residue, can either be (i) claimed as asphalt, or (ii) subject to further processing, such as coking. In various coking processes, the resid is heated to high temperatures of 850-950° F. (454-510° C.) such that the light boiling products are thermally cracked off of the aromatic cores in the resid and are distilled overhead and the solid coke remains.
The delayed coking process is one of the most widely commercially practiced of the coking processes. The resid is heated to the coking temperature by flowing through a long tube in a furnace and then allowed to react at this elevated temperature after flowing into the bottom of a high cylindrical insulated drum. The volatile products are removed to a fractionator and coke accumulates in the drum. The heavy liquid product from the fractionator is recycled back to the furnace. When the drum fills up with coke, the feed is switched to a second drum. The coke is mined out of the drum by drilling a hole down the center with high pressure water and cutting out the remainder also with high-pressure water to get the drum ready for the next coke accumulation cycle.
In Fluid Coking™, the resid is sprayed onto a hot, fluidized bed of coke particles in a vessel (i.e., the reactor). The volatile products are removed to a fractionator while the coke particles are removed from the bottom of the vessel and transferred to another vessel (i.e., the burner), where the coke is partially burned with air to provide heat for the process. The coke then is recirculated back to the reactor. Since this process produces much more coke than is required for heating the process, fluid coke is withdrawn at the bottom of the reactor.
In FLEXICOKING™, a third vessel (i.e., the gasifier), is added to the Fluid Coking process. In the gasifier, coke is gasified with steam and air in net reducing conditions to produce a low BTU gas containing hydrogen, carbon monoxide, nitrogen, and hydrogen sulfide. The hydrogen sulfide is removed using adsorption. The remaining low BTU gas is burned as a clean fuel within the refinery and/or in a nearby power plant.
Visbreaking is a low conversion thermal process used originally to reduce the resid viscosity for heavy fuel oil applications. Today, it often uses a resid that exceeds minimum heavy fuel oil specifications and converts just enough to obtain 15-30% transportation boiling range liquids and still have the heavy product meet heavy fuel oil specifications. Since this process cannot tolerate coke formation, it is required to be within the coke induction period that may limit conversion, rather than heavy fuel oil specifications. A visbreaker reactor may be similar to a delayed coker with a furnace tube followed by a soaker drum. However, the drum is much smaller in volume to limit the residence time with the entire liquid product flowing through. Alternatively, the entire visbreaker may be a long tube coiled within a furnace. Upsets cause coke to form and accumulate on visbreaker walls, which requires periodic decoking.
Refinery process furnaces are widely used to heat hydrocarbon feedstocks in a variety of services, for example, crude oil feed to an atmospheric tower, crude residuum from the atmospheric tower for feed to a vacuum tower, and the like. Perhaps the most severe service is the heating of feedstock to a delayed coker. While coke deposition can be a problem in any refinery process furnace, because of the high temperatures employed and the residual nature of the coker feedstock, there is a pronounced tendency for the formation of coke deposits on the inside wall of the radiant tubing through the coker preheat furnace and the vacuum tower furnace.
Regardless of service, the formation of coke deposits is not desirable. Coke deposits can lead to increased pressure in the tubes due to the restriction of flow, and to higher tube wall temperatures due to the insulative effects of the coke deposits. Both higher pressure and higher temperature lead to premature failure of the tubes. Furthermore, it is often necessary to periodically remove the tube from service and remove the coke deposits by burning off the deposited coke by oxidation with air or another oxidant that is passed through the tube at a high temperature. This periodic burn-off can result in severe thermal cycling, which also reduces the life of the tube.
The coker tube furnace is the heart of the delayed coking process. The heater furnishes all of the heat in the process. Typically, there are two to four passes per furnace. The tubes are mounted horizontally on the side and held in place with alloy hangers. Multiple burners are along the bottom of the radiant wall opposite from the tubes and are fired vertically upward. Tall furnaces are advantageous since the roof tubes are less likely to have flame impingement and overheating by both radiation and convection. Normally just the radiant section of the heater is used to heat the oil for a delayed coker. The upper convection section of the coker heater is used in some refineries to preheat the oil going to the fractionator or for other uses e.g., steam generation).
The radiant section tubes in a fired heater used in many refinery process units can experience fouling on the inside and/or outside of the tube surface. External tube fouling occurs when the heater is oil fired. During oil combustion solid particulate matter is formed containing carbon, sulfur and metals which are present in fuel oil. This particulate matter will over time collect on external tube surfaces. Fired heaters that heat crude and reduced crude usually experience the highest level of internal fouling. With these fluids, the fouling occurs due to (i) the presence of solids in the fluid, (ii) thermal cracking forming high molecular weight compounds and (iii) in situ corrosion products. All these materials can end up sticking to the tube wall and forming “coke”. Liquids lighter than crude can also form internal deposits. For example, fired heaters heating liquid naphtha can experience internal tube fouling due to corrosion products and/or polymerization reactions forming long chain molecules which stick to the tube wall. Internal tube fouling usually has a large impact on heater operation and thermal efficiency.
These formations/formulate/coke deposits can result in an increase in the radiant tube metal temperature (TMT). As coke forms inside the heater tube, an insulation barrier between the metal and the “colder” process fluid is formed, resulting in an increased TMT. If coking is allowed to occur without intervention, a tube rupture as a result of high TMT (due to lessened metal strength) is possible. To avoid this, heaters with internal coke deposits can be operated at reduced rates (and hence reduced efficiency and productivity) such that metallurgical constrains are not exceeded on the tubes and tube rupture is avoided. Heaters in fouling service are designed to accommodate a specified TMT increase above the clean tube condition. When that limit is reached steps must be taken to remove the foulant. Often this means the heater must be shut down for cleaning. A secondary effect of internal fouling is increased pressure drop, which limits capacity and throughput. Heaters in fouling service are also designed to accommodate a specified increase in pressure drop. In most cases, the TMT limit is reached before the pressure drop limit. When coke forms in the heater tubes, it insulates the inside of the tube which results in elevated temperatures on the outside of the tube. With good operational practices, coker furnace can be operational for 18 months before decoking of the tubes is needed. Depending on the tube metallurgy, when temperatures approach 1250° F. (677° C.) on the exterior skin thermocouple, the furnace must be steam spalled and/or steam-air decoked or cooled down and cleaned by hydraulic or mechanical pigging.
During normal use, the internal surfaces of the fired heater tubes are subject to carburization sulfidation, naphthenic acid corrosion and other forms of high temperature corrosion as a result of the prolonged exposure to the stream of heavy crude oil, resid and other petroleum fractions. Carburization is a form of high temperature degradation, which occurs when carbon from the environment diffuses into the metal, usually forming carbides in the matrix and along grain boundaries at temperatures generally in excess of 1000° F. (538° C.). Carburized material suffers an increase in hardness and often a substantial reduction in toughness, becoming embrittled to the point of exhibiting internal creep damage due to the increased volume of the carbides. Crude oils and hydrocarbon fractions which contain reactive sulfur are corrosive to carbon and low/medium alloy steels at temperatures above 500° F. (260° C.) and will cause sulfidation corrosion which forms iron sulfide. This sulfide scale that is formed is often referred to as sulfide induced fouling. Those which contain naphthenic acidic components are corrosive to carbon and low/medium alloy steels at temperatures above 400° F. (204° C.) and directly remove metal from the surface of the fired heater tube. Corrosion on the internal surfaces of the tired heater tubes creates an uneven surface that can enhance fouling because the various particles found in the petroleum stream may attach themselves to the roughened surface. It is also suggested that corroded surfaces may also provide a “more hospitable” surface for foulant lay down.
The radiant coil of the refinery process furnace has an inlet pipe section and an outlet pipe section. A plurality of essentially straight horizontal pipe sections is arranged in at least two vertical banks. The vertical banks are parallel and horizontally spaced apart. A plurality of bent pipe sweep return bends are arranged in vertical banks at either end of the straight pipe banks. Each bend connects a pair of straight pipe sections in adjacent vertical banks thereof. The return bends are sloped between horizontal and vertical, and one of the straight pipe sections in the pair connected by a return bend is elevated with respect to the other. A tubeside fluid flow path is provided from the inlet pipe section through an alternating series of the straight pipe sections and the return bends to the outlet pipe section. The coil preferably includes first and second vertical straight pipe banks and opposing return bend banks, wherein the straight pipe sections and the return bends are evenly spaced from adjacent sections and bends above and below, except for uppermost and lower most pipe sections and return bends. The return bends at either end of the adjacent tube banks can be oppositely sloped so as to provide a generally horizontal-helical flow pattern. The coil preferably has first and second nested passes, wherein the fluid flow paths of the first and second passes each comprise a series of alternating straight pipe sections in each of said vertical banks thereof, wherein the straight pipe sections of the first pass in the first and second banks are horizontally spaced opposite the straight pipe sections of the second pass in the respective second and first banks. The first and second pass straight pipe sections in each vertical tube bank can be alternated every other one from top to bottom.
A plurality of essentially straight horizontal radiant coils in the refinery process furnace, more specifically, in the vacuum tower furnace is made out of low chromium steels such as T9 and T5 for enhanced corrosion resistance, creep strength and rupture ductility. The typical composition of radiant coils used in the refinery process furnace is shown in Table 1.
TABLE 1Typical composition of radiant coils in the refinery process furnaceBase Metal, RAlloyUNS No.Alloy Compositions (Weight %)Carbon steels1018G10180Bal. Fe, 0.6-0.9Mn, 0.14-0.20C4130G41300Bal. Fe, 0.35-0.60Mn, 0.80-1.15Cr,0.27-0.34CLow chromiumT11K11562Bal. Fe: 1.25Cr: 0.5Mo, 0.5Si,steels0.3Mn, 0.15C, 0.045P, 0.045ST22K21590Bal. Fe: 2.25Cr: 1.0Mo, 0.5Si,0.3Mn, 0.15C, 0.035P, 0.035ST5S50100Bal. Fe: 5Cr: 0.5Mo, 0.5Si, 0.3Mn,0.15C, 0.04P, 0.03ST9J82090Bal. Fe: 9Cr: 1.0Si, 0.35Mn, 0.02C,0.04P, 0.045S
All the listed materials for refinery furnace tubes are either carbon steels or low chromium steels that contains chromium less than about 15.0 wt. %, preferably less than about 10.0 wt. %, based on the total weight of the steel. Corrosion protection of these materials relies on protective Cr7O3 films on the tube surface. However, chromium concentration in these steels are not sufficient enough to form such a protective film and rather forms a complex corrosion scale comprised of spinel and magnetite type oxide and sulfide. This corrosion scale leads to rough surfaces, high surface areas, and a large number of surface sites for the anchoring of coke and coke precursors.
The listed low chromium steels may contain small amounts of carbide formers such as vanadium, niobium and titanium for precipitation strengthening and/or grain refinement. These alloying elements also affect transformation hardening and weldability of the low chromium steels. The three general types of creep-resistant low chromium steels are Cr—Mo steels, Cr—Mo—V steels and modified Cr—Mo steels. The Cr—Mo steels are widely used in oil refineries, chemical industries and electrical power generating plants for tubing, piping, heat exchangers, super heater tubes, and pressure vessels. The main advantage of these steels is the improved creep strength from Mo and Cr additions and the enhanced corrosion resistance from Cr. The creep strength of Cr—Mo steels is derived mainly from two sources: solid-solution strengthening the ferrite matrix by carbon, molybdenum, and chromium, and precipitation hardening by carbides. Creep strength generally, but not always, increases with higher amounts of Mo and Cr. The effects of Cr and Mo on creep strength are rather complex. For example, T22 steel has higher creep strength than T5 steel.
Synthetic crudes are derived from processing of bitumens, shale, tar sands or extra heavy oils and are also processed in refinery operations. These synthetic crudes present additional fouling problems, as these feedstocks are too heavy and contaminant laden for the typical refinery to process. The materials are often pre-treated at the production site and then shipped to refineries as synthetic crudes. These crudes may contain fine particulate siliceous inorganic matter, such as in the case of tar sands. Some may also contain reactive olefinic materials that are prone to forming polymeric foulant deposits within the fired heater tubes.
Currently, there are various surface modification techniques available for reducing corrosion and fouling in the fired heater tubes for refinery operations. Most of them are based on thin film coatings and include alonizing, hexamethyldisilazane (HMDS) and liquid phase silicate coatings. Alonizing is a diffusion alloying method and applied to the metal surface at elevated temperatures. As a result, about 100μ thick, aluminum enriched layer forms on the metal surface. However, this coating, as characteristic of all such relatively thin coatings, reveals poor mechanical integrity and thermal stability due to presence of voids, defects and intermetallic brittle phases in the layer and has low reliability.
Therefore, there is a need to significantly reduce corrosion and fouling in the fired heater tubes in refinery processing operations that does not encounter the drawbacks associated with the current techniques.